The tissue diagnosis of cancer is routinely performed by standard surgical histo-pathological analysis, which involves preparation of tissue and then viewing under the microscope. This method is painful for patients, time consuming, and its results can take up to a few days to be ready. Direct visualization is also used, but is weakened by subjective decision. Currently, a frozen section diagnostic method is employed for operating room situations, where there is a need for immediate results. This technique can take up to 30 minutes. There are major difficulties with this method, primarily due to inaccurate diagnosis and a large amount of artifact in the tissue preparation. One of the most difficult issues involves finding a location having an appropriate surgical margin.
A number of systems have been developed in the past twenty years to perform biopsy on tissue by optical techniques. The early nineteen-eighties saw the development of simple systems which would irradiate either UV, visible, or infrared lights onto the tissue, and attempt to characterize the resulting reflectance or fluorescence emission spectra. This type of illumination was not always specific. Furthermore, high levels of tissue autofluorescence were observed. Therefore, there was a need to develop specific tumor markers such as Homoproto Porphorin Derivative (HPD). These substances were accordingly developed. HPDs are injected into tissue before laser illumination. After a suitable time, laser interrogation is performed. Problems of toxicity, patient safety, and convenience have prevented the widespread use of these photodynamic agents.
Later attempts to use laser induced fluorescence as a diagnostic tool resulted in the development of simple single fiber systems. Such systems illuminate the suspect areas of tissue with laser light at a focal point where a single fiber relays the tissue fluorescence. Actual spectral differences were found to exist between normal tissue and abnormal tissue. These studies have led to an array of spectral analysis technique, but they have all been limited to three or four spectral lines corresponding to reflectance of fluorescence signature peaks. The exact reason for the differences in normal versus cancerous tissue are not understood, but might be related to the three-dimensional structure or differences in biochemical makeup. The ability to exploit these differences can be used as a diagnostic tool.
Although these systems are relatively simple to use and can be adapted to existing endoscopic and colonoscopic instruments for measurements, they have three fundamental limitations:                1. The techniques can easily miss a small tumor since single fiber illumination area is extremely small.        2. The techniques cannot provide information on surgical margins during operative procedure due to lack of imaging capabilities.        3. Most techniques require the application of photodynamic agents.        
Mooradian et al., in U.S. Pat. No. 5,782,770 have proposed a technique for diagnosing tissue via hyperspectral imaging. In this technique, the spectral content of the image can be analyzed on a pixel-by-pixel basis to determine the presence of certain matter. Although this technique operates in real time and is non-invasive, it provides information only from the surface of the tissue. A real time three-dimensional tomography is needed to fully differentiate normal tissue from abnormal tissue.
Accordingly, a real time, non-invasive method is needed to rapidly diagnose the tissue, reduce the uncertainty of tissue diagnosis, and provide an actual image to identify the exact surgical margins during operative procedures.